This invention relates to semiconductor devices and methods of manufacture and more particularly to an improved fusible-link programmable read only memory.
Programmable read only memories or PROMs are used in microcomputers and similar digital systems. One of the types which is most widely utilized is the fusible-link PROM, wherein a small necked down portion of a metal conductor in each cell acts as a fuse and can be blown out by over-current selectively applied in order to program the memory. Usually the fusible-link is made of a thin film of a refractory metal such as titanium:tungsten. One of the problems with these devices is that the thin film is deposited on top of a coating of silicon oxide only a few tenths of a micron thick on a silicon substrate; the substrate acts as a heat sink so the heat generated by over-current in programming is conducted away. This requires much more current to be used than if a fuse of the same dimensions was freestanding and this current requires larger transistors on the chip. Another problem is that in the manufacture of transistors or diodes of the type needed in each cell of a PROM array and in the decoders and the like peripheral to the array on the chip, it is necessary to cover the semiconductor surface with a protective glaze to prevent deterioration. This glaze also covers the fusible-links and contains the melt when programming. This can cause unwanted shorts or low resistance.
It is therefore the principal object of the invention to provide an improved fusible-link PROM. Another object is to provide fusible-link PROMs which may be programmed with lower current, and which are more reliable.